Creating Sapling Tree in the Python Console
Using 'INVOKE_DEFAULT' Execution Context   to your operator call always creates default

bpy.ops.curve.tree_add('INVOKE_DEFAULT')


"Basically" this emulates the pressing (invoking) of button / menu in UI, and calls the invoke method, that in this case is required. This will create the default tree "quaking aspen"

Note using 'INVOKE' is calling the preset

bpy.ops.sapling.importdata(filename="callistemon.py") is hardcoded into the invoke method, changing other settings with 'INVOKE_DEFAULT' will have no effect.  Will always get this sucker when invoked.



Fixing to call from command line.

Edit: it appears the bend parameter has been not completely edited out of the addon.  . For now to fix, run the addon and right click on any operator property and "view source". This will open __init__.py from the addon in  your text editor. (Or go to addons/add_curve_sapling in filebrowser and open with text editor of choice) 

Search for bend around line 713 and remove the quotes """ from above and below the property definition.



EDIT

As of 2.83 still need to make edit above as well as commenting out line 1611 in utils.py

#cu.use_uv_as_generated = True


Save the text file, hit F8 to reload addons.
Or for 2.8 run 

bpy.ops.script.reload()


Now it will execute correctly with

bpy.ops.curve.tree_add(some_parameter=some_setting)


Test in 2.83 using command from question crunched into console I get



producing